1. Field of the Invention
The present invention relates to an image reading device used in equipment such as a scanner, a copying machine, or a facsimile, and an image forming apparatus comprising the image reading device.
2. Description of Related Art
Image forming apparatuses such as copying machines or facsimiles comprise an image reading device for reading a document image and an image recording device for recording on a paper sheet an image corresponding to the image read by the image reading device. The speed at which the image is read by the image reading device and the speed at which the image is recorded by the image recording device are generally set to approximately equal values. An image reading operation and an image recording operation are synchronously performed.
The image reading device comprises an image sensor represented by a CCD (Charge Coupled Device). The image sensor detects, when a light source is moved relative to an image surface of a document, light reflected from the image surface as incident light while illuminating the image surface by the light source. The image sensor converts the incident light into charge and stores the charge, and converts the stored charge into an electric signal and outputs the electric signal.
The electric signal is inputted to a laser scan unit in a copying machine, for example, as an image signal. The laser scan unit subjects laser beams to ON/OFF control depending on the image signal, and exposes a surface of a photoreceptor by the laser beams. The surface of the photoreceptor before the exposure is uniformly charged, so that an electrostatic latent image is formed on the surface of the photoreceptor upon exposure by the laser beams. The electrostatic latent image is thereafter developed into a toner image by a developing device, and the toner image is transferred onto the paper sheet.
Generally, the smaller the amount of light incident on the image sensor is, the smaller the amount of charge stored in the image sensor is, while the larger the amount of the incident light is, the larger the amount of the stored charge is. If the amount of incident light is insufficient, the amplitude of the image signal is decreased, so that the SN (Signal-to-Noise) ratio thereof is lowered. As a result, the image quality of the toner image is degraded. On the other hand, if the amount of the incident light is too large, an overflow of charge called blooming occurs, thereby making it impossible to perform normal image reading.
A light source for illuminating the document is degraded with an elapse of a time period during which it is used (a time period during which it is turned on) so that the amount of emitted light is decreased. Correspondingly, the S/N ratio of the image signal is reduced. A light amount adjusting function has been conventionally provided to the light source to adjust the amount of light emitted from the light source, thereby adjusting the amount of light incident on the image sensor in a suitable range.
In order to provide the light amount adjusting function to the light source, however, a light amount adjusting circuit or the like must be provided. Accordingly, the cost of the image forming apparatus is increased.
On the other hand, even when a light source having a light amount adjusting function is used, the amount of light incident on the image sensor cannot be always suitably adjusted. That is, there is a case where even if the amount of light from the light source is adjusted to a maximum, an amount of incident light required for the image sensor cannot, in some cases, be ensured. If a light amount adjusting range is narrow, it may be impossible to ensure a suitable amount of light incident on the image sensor.
An example of the image reading device provided in the image forming apparatus is one capable of setting a monochrome mode in which a document image is read in a single color and a full color mode in which a document image is read in a full color. Further, the image reading device in the image forming apparatus may, in some cases, be used as an image scanner for accepting an image in a personal computer or the like.
Since the image sensor capable of reading an image in a full color comprises a color filter, sensitivity to the amount of incident light tends to be low due to the effect. If a light source outputting a large amount of light, for example, a halogen lamp is used, the problem will be solved. However, such a high-power light source is expensive and consumes a large amount of power. Further, a rare gas lamp which has been frequently used in recent years as a light source for document illumination cannot obtain a sufficient amount of light. So long as the rare gas lamp is used, a sufficient S/N ratio at the time of reading a high-quality image in the full color mode is difficult to ensure.
A first object of the present invention is to provide an image reading device capable of improving the image quality of a read image without requiring to adjust the amount of light from a light source.
A second object of the present invention is to provide an image reading device capable of reading an image with its quality enhanced depending on an image reading mode.
A third object of the present invention is to provide an image forming apparatus capable of reading a document image with its quality enhanced without requiring to adjust the amount of light from a light source and forming an image corresponding to the read image on a paper sheet.
A fourth object of the present invention is to provide an image forming apparatus capable of reading a document image with its quality enhanced depending on an image reading mode and forming an image corresponding to the read image on a paper sheet.
An image reading device according to the present invention comprises an image sensor for scanning a surface of a document, optically reading an image formed on the surface of the document, and outputting an electric signal corresponding to the read image; a period control circuit for controlling the period of a horizontal synchronizing signal to be fed to the image sensor in order to change a time period during which charge is stored in the image sensor; and a scanning speed control circuit for controlling the scanning speed for the surface of the document by the image sensor to be a speed conforming to the period of the horizontal synchronizing signal.
The image reading device according to an embodiment of the present invention further comprises an image reading mode setting section for setting an image reading mode. In this case, it is preferable that the period control circuit variably sets the period of the horizontal synchronizing signal depending on the image reading mode set by the image reading mode setting section. Similarly, it is preferable that the scanning speed control circuit variably sets the scanning speed depending on the image reading mode set by the image reading mode setting section.
The scanning of the surface of the document by the image sensor may be attained by causing the document to stand still on a transparent platen, for example, while moving a light source for illuminating the document relative to the document as well as causing light reflected from the document to be incident on the image sensor. Further, the scanning of the surface of the document may be attained by causing the light source for illuminating the document to stand still, while conveying the document such that the document passes through a position illuminated by the light source and causing the light reflected from the document to be incident on the image sensor in the course of conveying the document. Consequently, the adjustment of the scanning speed for the surface of the document by the image sensor is attained by adjusting the speed of a motor for moving the light source or a motor for conveying the document (hereinafter generically referred to as a scan motor), for example.
It is preferable that the image reading mode which can be set by the image reading mode setting section includes a monochrome mode and a full color mode. Further, the image reading mode which can be set by the image reading mode setting section includes a monochrome mode, a full color mode, and an image scanner mode. In such a case, it is preferable to set the period of the horizontal synchronizing signal to a relatively short period when the image reading mode is the monochrome mode, while setting the period of the horizontal synchronizing signal to a relatively long period when the image reading mode is the full color mode or the image scanner mode. Consequently, at the time of the full color mode or the image scanner mode, a time period during which charge is stored in the image sensor is lengthened. Accordingly, a high S/N ratio is ensured, thereby making it possible to read an image with its quality enhanced. On the other hand, at the time of the monochrome mode, a time period during which charge is stored in the image sensor is shortened, thereby making it possible to read an image at high speed.
In conformity with such setting of the horizontal synchronizing signal, it is preferable that the scanning speed control circuit sets the scanning speed to a relatively high speed when the image reading mode is the monochrome mode, while setting the scanning speed to a relatively low speed when the image reading mode is the full color mode or the image scanner mode.
More specifically, the period of the horizontal synchronizing signal at the time of the monochrome mode, for example, may be set to an approximately integral fraction of 1 (for example, one-half) the period of the horizontal synchronizing signal at the time of the full color mode or the image scanner mode. Correspondingly, the scanning speed at the time of the monochrome mode may be set to an approximately integral multiple of (for example, twice) the scanning speed at the time of the full color mode or the image scanner mode. Such setting of the period of the horizontal synchronizing signal or the scanning speed can be attained by choosing whether or not an output of a reference signal generation circuit for generating a reference signal is frequency-divided. Accordingly, it is possible to adjust the period of the horizontal synchronizing signal and adjust the scanning speed in simple construction.
An image reading device according to another embodiment of the present invention further comprises a light amount sensor for detecting the amount of light from a light source for illuminating the surface of the document, and a light amount comparison circuit for comparing the amount of light detected by the light amount sensor with the reference amount of light. In this case, it is preferable that the period control circuit variably sets the period of the horizontal synchronizing signal on the basis of the results of the comparison by the light amount comparison circuit. Consequently, a time period during which charge is stored in the image sensor is variably set depending on the amount of light from the light source. Accordingly, an image can be read with its quality enhanced without being affected by the variation in the amount of light from the light source and the change with time of the amount of light from the light source. It is preferable that the scanning speed of the document is variably set depending on the results of the comparison by the light amount comparison circuit in response to the variable setting of the horizontal synchronizing signal.
Furthermore, the results of the comparison by the light amount comparison circuit may be displayed on the display device, and a maintenance worker may variably set the period of the horizontal synchronizing signal and the scanning speed manually depending on the results of the comparison.
The light amount sensor may be used as the image sensor. The light amount sensor may be provided separately from the image sensor.
An image forming apparatus according to the present invention comprises the above-mentioned image reading device, and a print section for forming on a paper sheet an image corresponding to the image read by the image reading device. Such construction makes it possible to read a document image with its quality enhanced and correspondingly to form the image of high quality on the paper sheet.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.